Direct digital synthesizers (DDS) can generally be characterized as having either phase continuous switching behavior or phase coherent switching behavior. With phase continuous switching behavior, there are no large phase discontinuities when the synthesized analog signal output from the DDS is switched between different frequencies. Phase continuous switching is the usual DDS switching behavior during a frequency change. Phase coherent switching, which is also known as phase repeatable switching, applies a correction to the phase accumulator in the DDS at each frequency change, so that when the synthesized analog signal output from the DDS is switched back to a given frequency, the phase of the synthesized analog signal will be the same as if the synthesized analog signal had never been switched from the given frequency. Phase coherent switching may be used to simulate numerous independent synthesizers each outputting an analog signal at respective different frequencies.
However, to correct and appropriately place the phase of the synthesized analog signal, phase coherent switching typically requires a phase delta to be calculated based on the current state of the DDS every time the frequency of the synthesized analog signal is switched. A phase delta may be calculated and added to the current accumulator value of the DDS based on the previous frequency and the new frequency of the synthesized analog signal. The current accumulator state can thus be said to have a memory of all transitions that have occurred since the last time the DDS was at a given frequency. If the current accumulated phase value of the DDS is not corrected at each transition of the synthesized analog signal to a new frequency, the memory of how to return the synthesized analog signal to a previous frequency will be lost. Such a typical implementation does not allow DDS operation intermixed between phase coherent switching and phase continuous switching.
There is therefore a need to provide a DDS selectively operable and switchable in a phase coherent mode and a phase continuous mode.